Metalaxyl is the active ingredient in fungicides such as Ridomil, Apron, Subdue, and others used to prevent root rots and seedling diseases caused by the fungus-like organism Pythium. Called Oomycetes, these fungus-like organisms require water for a portion of their life cycle, because most produce a swimming spore and are more closely related to brown algae than to true fungi like stripe rust. Other common oomycetes are the downy mildew and the late blight pathogen on potato. Pythium is a soilborne pathogen present in most agricultural soils that is able to attack a diversity of crops grown in the PNW including wheat, chickpeas (Chen and Van Vleet, 2016), lentils, canola, potatoes (Porter, et. al, 2009), other vegetables, and even the tree fruit. Because Pythium is not a true fungus, only certain fungicides can be used to protect a crop with, metalaxyl being most frequently used, most often in the form of a seed treatment. Unfortunately, in both potato-producing regions and in chickpea production in the Palouse, metalaxyl-resistant Pythium have been found. These resistant Pythium species are able to cause damping-off, stand and crop loss, and leak (in potatoes) despite the seed treatments. The fungicide ethaboxam is proposed as an alternative for managing metalaxyl resistant Pythium populations. As metalaxyl is our main weapon against Pythium and other oomycetes, it is vital that we be aware of developing resistance so that we can manage these populations and slow further development. Changes in management practices that encourage the rapid growth of seedlings and reduces cool, wet soil conditions until plants are robust enough to withstand minor damage can also help reduce the impact of Pythium.
If you suspect that you may have metalaxyl resistant Pythium, you are encouraged to submit a soil or plant sample to the Plant Pest Diagnostic Clinic in Pullman for testing.
For more information:
Weidong Chen and Steve Van Vleet. Chickpea damping-of due to metalaxyl-resistant Pythium: an emerging disease in the Palouse. 2016. http://hdl.handle.net/2376/6273
Cook, R.J. and B.X. Zhang. 1985. Degrees of sensitivity to metalaxyl within the Pythium spp. pathogenic to wheat in the Pacific Northwest. Plant Disease 69: 686-688.
Porter, L.D., P.B. Hamm, N.L. David, S.L. Gieck, J.S. Miller, B. Gundersen, and D.A. Inglis. 2009. Metalaxyl-M-resistant Pythium species in potato production area of the Pacific Northwest of the U.S.A. American Journal of Potato Research 86: 315-326.